Overhead line structure footing resistance measurements 1

[KillBill7]

We are not a utility, but we manage a large network of distribution and transmission lines that feed critical loads. Recently, we experienced some lightning-related outages on our 34.5kV circuits. Upon investigation, we found that the footing resistance of the structures was quite high, leading to backflashovers.

I’m curious about the merits of conducting footing resistance measurements regularly to ensure that the resistance values remain within the required standards. For those with a utility background, what is the typical practice? Are footing resistances measured on a periodic basis for all circuits, regardless of the voltage level, or is this done on an as-needed basis?

 

[waross]

Concrete footings?
Steel pile footings?
For lightning issues, does the impedance differ from the resistance appreciably?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[KillBill7]

Most of our steel poles have concrete footings. We have some old wood poles in our systems, which are directly buried. The measured footing resistance was above 100 ohm on affected structures.

 

[Parchie]

@KillBill7,
We have the same problem in my other life. What we did was drive additional ground rods at our transmission towers. We measured earth resistance after each activity and make sure our earth resistances don't exceed 25 ohms. Also, we found out those copper conductors we used were like magnets for copper thieves! Just so you'll know!

 

[davidbeach]

Use copperweld, not copper. Then paint it silver. Since it doesn't cut like copper they're more likely to give up before the first one is cut through.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[waross]

Connecting to the rebar creates a very effective Ufer ground.
Question: Has anyone experienced concrete damage when lightning hits a Ufer ground?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[KillBill7]

Thanks for the suggestions everyone. I guess what I was trying to ask if there is a merit to do the periodic ground resistance measurements for the structures like every 5 years or so. Visual inspection would able to identify any missing ground links due to theft etc. However, things like change in soil composition, corrosions of ground rods would also impact the footing resistance so I am wondering if the utilities perform these measurements on periodic basis as part of their preventive maintenance program.

 

[Heaviside1925]

OP,
I do not think it would be much merit metering the grounds on a periodic cycle. I would however get a baseline for the structures if you do not have one already and flag the ones that need possible remediation. Other posters have provided possible remediation steps.
My experience (mostly heavy industrial in terms of transmission) Have I found existing grounds to not be sufficient? yes. Have I had questionable grounds deteriorate to the point of requiring remediation? yes. Have I had properly remediated or installed grounds become questionable or deficient? yes but there was always a reason such as a large fault or lightning strike. If you know all of your grounds are well within spec, then I would install a tattletale of some sort at the base that your crew could visually inspect to indicate a fault or lightning strike having taken that path. Then you could meter that ground to determine of it was still in spec.

 

[wcaseyharman]

I do not believe my utility checks for structure-ground resistance outside of substation ground grids. Their practice is to place a ground plate at every structure.

The only time I’ve come across this question is related to elevated neutral voltage on unbalanced/single phase circuits. It seems the utility relies on the large number of grounds (substation grid, each pole ground, plus customer premises grounds). However, the utility I work for has very low lightning exposure.